1. Field of the Invention
The present invention relates to the field of radio antennas, and more particularly, to wide frequency coverage vertical, dipole and parasitic array antennas.
2. Description of the Related Art
It is often desired to provide a single antenna having excellent performance over a wide frequency range. In the interest of efficiency and impedance matching, antennas used for radio communication are generally resonant antennas. Unfortunately, resonant antennas by their nature operate over a very narrow range of frequencies. To be resonant at a specific frequency, the antenna must be a certain specific length.
Three commonly used resonant antennas are the dipole, vertical and Yagi-Uda. A dipole antenna is comprised of a single element, usually one half of a wavelength long at the design frequency. It is then usually split at the center where electromagnetic energy is then fed. Vertical antennas are basically dipoles oriented in a vertical plane with one half of the element being driven and the other half removed. The earth is then used as a conductor in its place. Yagi-Uda antennas, frequently referred to as parasitic arrays, are known in the art to provide directional transmission and reception with a high front-to-back ratio as well a low VSWR throughout a very narrow band of contiguous frequencies. Most embodiments of a Yagi-Uda antenna use a single element that is driven from a source of electromagnetic energy. Arrayed with the driven single element are the so-called reflector and director elements that are not driven directly, known as parasitic elements. There is usually only one reflector and one or more directors, with the favored direction of transmitting and reception towards the director elements.
The Yagi-Uda antenna is basically a single frequency device that can be designed to work satisfactorily over a few percent of the center design frequency. However, tradeoffs must be made between gain, front-to-back ratio, and VSWR to allow the antenna to work over this very narrow 3%-4% range. It is often desirable to have a single Yagi-Uda antenna operate in multiple frequency bands. Many radio services have assigned frequencies segregated into bands scattered through the radio spectrum. The amateur radio service is a good example of this, having bands approximately centered at 160M, 80M, 40M, 30M, 20M, 17M, 15M, 12M, 10M, 6M, 2M, etc. Radio amateurs commonly use Yagi-Uda arrays in the 40 m and higher bands. Some prior art antenna designs address multiple bands that cover three of the aforementioned bands, and in some cases five bands, but with very compromised performance. To provide even marginal performance, these antenna designs require large and complex arrays.
To enable wider frequency coverage, three methods have been classically employed. A common method is the use of xe2x80x9ctrapsxe2x80x9d that allow one element to function on three bands. Traps are parallel-resonant circuits placed at specific locations on the element to decouple a portion of the element automatically as the antenna operation is changed from band to band. Although multi-element trapped antennas cover multiple frequencies with fewer elements than others designs, they cannot be optimally tuned and there are significant losses associated with traps in all of the elements including the driven element. A trapped Yagi-Uda antenna is a significant compromise in gain, front-to-back ratio, and overall efficiency.
Another method to obtain wider frequency coverage is the use of a so-called log-periodic antenna, in which every element is driven and no element is parasitically driven. This type of antenna can operate over a range of frequencies having a ratio of 2:1 or higher. The antenna impedance varies logarithmically so the VSWR can range as high as 2:1. The log-periodic antenna trades off wide bandwidth for gain and front-to-back ratio. The log-periodic antenna has less gain and less front-to-back ratio than a three element monoband Yagi-Uda antenna yet requires many more elements and a complex feed system.
Yet another method of obtaining wider frequency coverage is the use of an open-sleeve cell type of driven element. This method uses one or more parasitically excited elements placed very close to the driven element. The length of these parasitic elements is usually half that of the driven element. This method results in a wider VSWR bandwidth and the ability to operate on two different frequencies with a single feedline. However, the open-sleeve technique only applies to a driven element. Yagi-Uda antennas require additional dedicated parasitic elements for each anticipated frequency band.
It is an object of the present invention to provide a tunable antenna system with at least one driven element that can be selectively adjusted in length to receive and transmit different frequencies.
It is another object of the present invention to provide such an antenna system that can be used with parasitic elements.
It is a further object of the present invention to provide such an antenna system that is easy to assemble and dismantle.
Disclosed herein is an antenna system comprising of an antenna with at least one driven element made up of two longitudinally aligned support arms joined at their proximal ends to a rigid housing unit affixed or mounted to a boom or support pole. Disposed inside the two support arms are two length adjustable conductive members that are electrically separated to form a dipole or connected together to form a parasitic element. Disposed inside the housing unit is a means for adjusting the length of the two conductive members inside the support arms. In the preferred embodiment, the means for adjusting the length of the conductive members are two spools located inside the housing unit in which the conductive members are wound. During use, one conductive member is associated with one support arm and is selectively wound and unwound from a spool so that the conductive member moves longitudinally inside the support arm. At least one motor is provided inside the housing unit that rotates the spools to precisely control the length of the conductive members inside the support arms. In one embodiment, the support arms are rigid and fixed in length. In a second embodiment, the support arms are telescopic and capable of being adjusted in length.
The antenna system also includes a radio system that is connected to the driven element on the antenna. The antenna system may have one or more parasitic elements. The system also includes an electronic control unit that controls the length of the conductive member in each element on the antenna which allows the operator to select a desired frequency, read the operating frequency of the radio, adjust the antenna manually or automatically or measure the transmit frequency with a frequency counter, and then adjust the antenna automatically. In a second embodiment, both support arms are telescopic and adjustable in length. The distal ends of the conductive members are attached to the distal ends of the support arms so that the overall size of the antenna may be adjusted when a desired frequency is received.
The above antenna system is especially advantageous when configured as a Yagi-style antenna that can be optimally tuned at a specific frequency for maximum gain, maximum front-to-back ratio, and to provide a desired feed point impedance at the driven element. This allows a very large continuous range of frequencies to be covered with excellent performance and a very low voltage-standing-wave-ratio (VSWR) while using only one feed line. By using length adjustable elements and a shorter boom, the antenna system is able to achieve better performance than prior art antenna designs. Also incorporated into it is a Yagi-style antenna, enabling it to be quickly adjusted to change the direction of maximum signal strength 180 degrees by changing the length of the designated director to make it function as a reflector and conversely changing the length of the reflector to make it function as a director. In should also be understood that the antenna system can also function as a bi-directional style antenna by adjusting the reflector element to function as a director.
An electronic control system is provided that manually or automatically adjusts the length of the conductive members inside the antenna driven and parasitic elements to receive or transmit a desired frequency.